This non-provisional application claims priority under 35 U.S.C. § 119(a) on Korean Patent Applications Nos. 2003-18165 and 2003-87110 filed on Mar. 24, 2003 and Dec. 3, 2003, respectively, which are herein incorporated by references.
1. Field of the Invention
The present invention relates to an enhancement of the optical and mechanical properties in a polymeric optical element by an annealing process performed under a compressed gas, and more particularly to a method of improving optical and mechanical properties of a polymeric optical element by annealing a polymeric optical element using a compressed gas under supercritical conditions.
2. Description of the Related Art
Examples of optical elements made of polymeric materials, include polymeric plastic optical fibers, optical waveguides, micro-mirrors, lenses, light guide panels of liquid crystal displays, diffusers, and holographic optical elements (HOE), which are for use in the field of displays (refer to “Polymers for Waveguide and Integrated Optics: Technology and Applications”, by R. A. Horank, Marcel Dekker (1992); “Liquid Crystal Devices: Physics and Applications”, by V. G. Chigrinov, Artech House (April, 1999); and “Polymers for Photonic Applications I. Nonlinear Optical and Electroluminiscence Polymers”, by C. Bosshard et al., Springer Verlag (March, 2002)).
In the production of polymeric materials, disadvantageous molecular orientation or residual stress is frequently experienced due to thermal history or flow history caused during the polymerization and molding process (refer to “Rheology: Principles, Measurements and Applications”, by Ch. W. Macosko, John Wiley and Sons (1994)). As a general solution for removing the undesired molecular orientation or residual stress, there has been proposed to conduct an annealing process at high temperature. This method, however, may cause deterioration in physical properties due to deformation or degradation of the elements. Especially, communication plastic optical fibers having a long light transmission path are intensely affected by the above disadvantages.
In general, optical loss of plastic optical fiber is relatively higher than that of quartz based optical fiber, and this is mainly caused by C—H absorption inside the polymers. Such optical loss largely depends on wavelengths of an optical source. In the case of PMMA(poly(methyl-methacrylate)), the optical loss theoretically exceeds 70 dB/km at a wavelength of 650 nm. Also, the optical loss resulting from Rayleigh scattering due to density fluctuation is more than 10 dB/km. In addition to the above-described intrinsic optical losses, there may be extrinsic optical losses resulting from exterior factors caused during fabrication such as, for example, the impurity of unreacted monomers. Summing up the intrinsic and extrinsic optical losses described above, the overall optical loss in plastic optical fibers generally exceeds at least 150 db/km.
The plastic optical fiber is generally fabricated through extrusion or preform drawing. In the case of a graded index type plastic optical fiber, having a refractive index gradually varied in a radial direction, the generally used fabrication method is that a cylindrical polymer rod, namely, a preform having a desired refractive index distribution, is thermally drawn while being heated in a furnace (refer to “Plastic optical fiber: An Introduction to Their Technological Processes and Applications”, by J. Zubira and J. Arrue, Optical Fiber Technol., pp. 101-140, vol. 7(2001)). During the thermal drawing of the plastic optical fiber, the furnace temperature, the preform input speed, and the optical fiber drawing speed should be properly controlled. If these conditions are not appropriately controlled, a high drawing tension may be produced in the final plastic optical fiber, resulting in an increase in the optical loss. When plastic optical fibers are annealed at a temperature near or higher than the glass transition temperature, the residual stress of the fibers is removed and the length of the fibers is shortened. (refer to “High Temperature Resistant Graded-Index Polymer Optical Fiber”, by M. Sato et al., J. Lightwave Technol., pp. 2139-2145, vol. 18(2000)). This means that a great deal of polymer chain orientation is induced during the drawing of the plastic optical fiber. Although there are substantially few reports as to how the polymer chain orientation affects optical properties of the plastic optical fiber, it is generally well known by those skilled in the art that when a preform having a high molecular weight is fabricated and thermally drawn, the result is a large amount of optical loss. In addition, if the preform is drawn at high speed in order to enhance the productivity of thermal drawing, the result is an increase of the drawing tension, thereby making it difficult to fabricate a high performance plastic optical fiber. Therefore, it can be said that the molecular weight range, which enables a plastic optical fiber having excellent mechanical and optical properties to be fabricated with high productivity, is narrower than the thermal drawing possible molecular weight range.